1. Field Of The Invention
The invention relates to the field of unsaturated polyester resins (which are indicated in the following text by the symbol UPR) and, in particular, to the field of substances which can be used in accelerating and crosslinking monomers in the polymerization (or curing) of UPRs.
2. Background Art
UPRs, in their most common form, consist of the following, as is known, for example in Ullmann's Enzyklopadie der techn. Chemie, 4th edition, Vol. 19, pages 79 to 88.
(a) an unsaturated polyester resin, commonly called alkyd, obtained in polycondensation reaction by reacting under appropriate conditions one or more glycols with one or more carboxylic acids or anhydrides thereof, of which at least one is a dicarboxylic acid (or anhydride) ethylenically unsaturated in the .alpha.,.beta.-position relative to the carboxyl groups. In general, the alkyd represents 40 to 90 percent of the UPR.
(b) One or more vinyl monomers which act as a solvent and at the same time as a crosslinking agent for the alkyd, in a concentration of 10 to 60 percent relative to the UPR.
(c) Polymerization inhibitors and/or stabilizers which allow the UPR to be handled and transported under safe conditions. These substances are added to the resin in a proportion of 50 to 1000 ppm.
(d) Technological additives of various types, incorporated in the UPR, in order to make it suitable for specific applications (concentrations of about 0.5 to 2 percents.
(e) Substances (commonly called accelerators or promoters) which are capable of appropriately modifying the polymerization kinetics of the UPR, with a view to its end use (concentrations of about 0.05 to 1 percent).
Although the substances listed under (a), (b) and (c) are always present in the UPR, the substances listed under items (d) and (e) may or may not be present, as the case may be.
The vinyl monomers [mentioned under item (b) above] are products which, when appropriately activated by an addition of catalysts, reacts with the double bond of the .alpha.,.beta.-unsaturated dicarboxylic acid incorporated in the alkyd. The reaction which takes place is a crosslinking reaction, also commonly called curing, which leads to a crosslinked product, whose chemical, physical and mechanical characteristics depend on the raw materials of the UPR and on the curing conditions.
Among the vinyl monomers most commonly used hitherto for curing UPRs, the following are mentioned: styrene, .alpha.-methylstyrene, p- or m-methyl-styrene, divinylbenzene, diallyl phthalate and its prepolymers, diallyl isophthalate, diallyl terephthalate and its prepolymers, diallyl isophthalate, diallyl terephthalate, N-vinylpyrrolidone, triallyl cyanurate, diallylmelamine and the like, by themselves or in various mixtures with one another. Other vinyl monomers which can be used are alkyl-styrenes and other allyl, acrylate or methacrylate esters. Although the most widely used of the vinyl monomers has been styrene, it causes problems of polluting the work environment, so that legislation in various countries is becoming increasingly restrictive with regard to its use.
Various products such as metal salts, for example, of cobalt manganese, vanadium and iron, tertiary aromatic amines, such as, dimethylaniline, dimethyl-p-toluene, diethyl-aniline, phenyldiethanolamine and the like have hitherto been used as accelerators or promoters of the curing reaction [regarding these, see item (e) above].
These known accelerators have not been found to be free of disadvantages, such as, in particular, a relatively low efficiency thereof, which required elevated temperatures and/or longer curing times in certain cases. Thus, for example, such accelerators gave complete polymerization of the UPR only when hot, that is, at temperatures from 50.degree. to 180.degree. C. Moreover, such accelerators require relatively long crosslinking times, which are inadequate for use on modern machines which, for their high productivity, require ever faster curing reactions.
U.S. Pat. No. 3,574,787 further discloses acetoacetic esters as additional accelerators for unsaturated polyester systems. These conventional acetoacetic esters such as the acetoacetic esters of mono- or di-functional alcohols also show a relatively low efficiency, but, in particular, they do not act as crosslinking monomers and therefore remain unchanged in the polymer lattice. Their concentration is therefore limited to about 1 percent by weight relative to the polymer composition because otherwise the properties of the polymer are unfavorably influenced.